In recent years, flat displays for information equipment are remarkably spread. In a liquid crystal display among these, light from a backlight is switched by an optical shutter function of a liquid crystal, and color is provided by using a color filter. An organic EL display (or an organic LED display), on the other hand, emits light from each pixels (i.e., self-luminous), and thus not only has an advantage of a wide viewing angle, but also has such many other advantages in that the device can have a low-profile owing to the disuse of a backlight, the display can be formed on a flexible substrate, and the like. Accordingly, an organic EL display is expected as a next-generation display.
The driving mode of the display panels are roughly classified into two modes. The first driving mode is referred to as a passive matrix type (or a duty driving mode or a simple matrix mode). In this mode, plural stripe electrodes are combined as rows and columns in a matrix form, and pixels positioned at intersecting points of the row electrodes and the column electrodes are made luminescent with a driving signal applied to the row electrodes and the column electrodes. The signal for controlling luminescence is generally scanned on row-by-row in time series, and applied simultaneously to all the columns of one row. This is a mode, in which no active device is generally provided on the pixels, which are controlled to emit light only in a duty period of the every row within the scanning cycle of the rows.
The second driving mode is referred to as an active matrix type, in which each pixel has a switching device, and is made luminescent over the scanning cycle of the rows. For example, such a case is assumed in that the entire surface of a panel having 100 rows and 150 columns is made luminescent at a display brightness of 100 Cd/m2. In this case, since the pixels in the active matrix type generally emit light continuously, it is sufficient that they are made luminescent at 100 Cd/m2 provided that the area ratio of the pixels and various losses are not taken into consideration. However, in order that the same display brightness is to be obtained with the passive matrix type, the duty ratio for driving the pixels is 1/100, and the luminescent period is only the duty period (selected period), whereby the luminescent brightness within the luminescent period is necessarily 100 times, i.e., 10,000 Cd/m2.
For increasing the luminescent brightness, the electric current applied to the luminescent device may be increased. However, it has been known that when the electric current is increased, for example, in an organic EL luminescent device, the luminescent efficiency is decreased. Upon comparing the active matrix type driving mode and the passive matrix type driving mode with the same display brightness, the consumed electric power is relatively larger in the passive matrix type due to the decrease in efficiency. Furthermore, upon increasing the electric current applied to an organic EL device, the materials are liable to be deteriorated due to heat or the like, which brings about a disadvantage of decreasing the service life of the display device. In the case where the maximum electric current is restricted from the standpoint of the efficiency and the service life, on the other hand, it is necessary to prolong the luminescent period for obtaining the same display brightness. However, since the duty ratio, which determines the luminescent period in the passive matrix type driving mode, is the inverse of the row number of the panel, the prolongation of the luminescent period leads to restriction in display capacity (driving line number). In view of these factors, it is necessary to use the active matrix type driving mode for realizing a panel with large area and high definition. For the ordinary active matrix driving, a mode using a thin film transistor as a switching device has been known.
In the active matrix type driving mode, which is suitable for large area and high definition, a thin film transistor (TFT) using polysilicon is most widely used as a switching device of pixel. However, there is such a problem, for example, that the process temperature for forming TFT using polysilicon is a high temperature of at least 250° C., and a flexible plastic substrate is difficult to use.
In order to avoid the various problems associated with the conventional display panels, the use of an organic thin film transistor device has been proposed.
For example, JP-A-2001-250680 (Patent Document 1) discloses that an organic thin film rectifying device is connected in series to an organic thin film luminescent part, and WO01/15233 (Patent Document 2) discloses that a pixel is controlled for driving with an organic thin film transistor. According to the disclosure of Patent Document 2, since the driving device is constituted by an organic material, the production process can be carried out at a low temperature, and thus a flexible plastic substrate can be used. Furthermore, inexpensive material and process can be selected for reducing the cost.
Patent Document 1: JP-A-2001-250680
Patent Document 2: International Publication 01/15233
Non-patent Document 1: T. Arai, Y. Fujisaki, T. Suzuki, et al., “Gate length dependency of characteristics of bottom contact type organic TFT on plastic substrate”, Preprints of 52nd Annual Meeting of the Japan Society of Applied Physics (March of 2005), p. 1511, upper columnNon-patent Document 2: N. Yoneya, T. Kimura, Y. Hirai, et al., “Organic TFT-driving QQVGA liquid crystal display on plastic substrate”, Preprints of 66th Annual Meeting of the Japan Society of Applied Physics (September of 2005), p. 1178, lower column
However, the organic thin film transistor has the following problems.
As disclosed in Non-patent Document 1, good FET characteristics for mobility, on/off ratio and threshold value are obtained only with a device having a long channel length. Upon producing a device using an organic thin film transistor, however, it is necessary to develop a device having a short channel length for reducing the size of the device.
Non-patent Document 2 discloses a technique for improving the state of a semiconductor/insulating film interface by using a hydrophobic organic insulating film, but even in this case, the mobility is 0.18 cm2/Vs (channel length=5 μm), which is only improvement about three times.
In view of the aforementioned points, accordingly, an object of the invention is to provide a process for producing a thin film transistor capable of preventing the FET characteristics from being deteriorated with a device having a short channel length.